The present invention relates to a disconnect contact assembly for connecting and disconnecting electrical equipment inside an equipment compartment, with power source conductors and a load conductor in a bus compartment.
FIG. 1 shows the configuration of a typical conventional enclosed switchboard. Referring to FIG. 1, inside a switchboard box 1, partition walls form an equipment compartment 2, a power source compartment 3, and a load compartment 4. Front doors 5a and 5b are arranged for the equipment compartment 2 and the power source compartment 3, respectively, in such a manner that they are free to open and close. Power source buses 6 are disposed inside the power source compartment 3, and a load conductor 7 is disposed inside the load compartment 4. A current transformer 8 is connected to the load conductor 7 which is connected to a load cable 10 through a cable head 9. A circuit breaker 11 is housed inside the equipment compartment 2 to be in electrical contact with the power source buses 6 and the load conductor 7 through upper and lower disconnecting switches 12 and 13 mounted to the partition walls defining the equipment compartment 2 and the compartments 3 and 4. FIG. 2 shows the electrical connections of the respective parts shown in FIG. 1. In FIG. 2, same reference numerals denote the same parts as in FIG. 1.
When the circuit breaker 11 is drawn outside the enclosed switchboard of this type, the upper and lower disconnecting switches 12 and 13 become exposed. In such a case, an engineer may come in contact with the exposed disconnecting switches, which presents a danger. In order to solve this problem, in an enclosed switchboard of this type, when the circuit breaker 11 is drawn outside the switchboard, a shutter device covering the upper and lower disconnecting switches 12 and 13 in synchronism with the drawing operation is generally incorporated. This shutter device is of metal for the purpose of safety for the engineer.
In the metal shutter device, shutter plates 14 and 15 are at the ground potential. Therefore, when the shutter plates 14 and 15 are at the "closed" position, a clearance of insulation "X" between stationary arms 16 and 17 and the shutter plates 14 and 15 must satisfy the insulating level of the enclosed switchboard. Reduction in the clearance of insulation "X" therefore is an important factor in reducing the outer dimensions, especially the depth of the enclosed switchboard.
The clearance of insulation "X" as described above is determined by the construction of the disconnecting switches 12 and 13. FIG. 3 is an enlarged view of portion "A" of FIG. 1. FIG. 4 is a sectional view of the shutter plate 15 in the "closed" position. As shown in FIG. 3, a plurality of fingers 19 are mounted to a finger arm 18 of the circuit breaker 11 (FIG. 1) as a movable arm, so as to electrically connect the stationary arm 16 and the finger arm 18 through the fingers 19. For the purpose of smooth attachment and detachment of the fingers 19 to the stationary arm 16, an end part 20 of the stationary arm 16 is usually processed to have an R shape. In the case of the "closed" position of the shutter plate 15 shown in FIG. 4, the air break distance between the stationary arm 16 and the shutter plate 15 is defined as the clearance of insulation "X". As has been described hereinabove, the end part 20 of the stationary arm 16 is processed into an R shape for the purpose of achieving smooth attachment and detachment of the stationary arm 16 to the fingers 19. In addition to this, from the viewpoint of withstand voltage characteristics, rounding generally results in a reduction of the convergance of the electric field, and improvements in the utilization efficiency of the electric field and the withstand voltage characteristics. For this reason, theoretically, an electrode processed in R shape has a smaller clearance of insulation "X" than that of a rod electrode which is not processed into R shape for use as the stationary arm. However, this R shape usually has a radius of about 5 to 10 mm. When the clearance of insulation "X" is 150 to 300 mm, the processing in R shape of this range does not improve the withstand voltage characteristics, and reduction in the clearance of insulation "X" by processing in R shape may not be expected. When the shutter plate 15 is at the "closed" position, the electric field converges on the end part 20 of the stationary arm so that a streamer is generated, resulting in flashover.